The purpose of this project is to study immune responses in man and experimental animals to natural and experimental rickettsial infections. Currently, studies have involved isolation and characterization of Coxiella burnetii isolates from heart valves and the characterization of the immune response in man to chronic Q fever disease. The various serological parameters were compared by the microagglutination, complement fixation and the indirect microimmunofluorescent antibody tests. These serological tests were found to adequately differentiate primary, granulomatous hepatic and chronic endocarditis Q fever disease entities in man. An enzyme-linked immunosorbent assay (ELISA) test has been developed for the diagnosis of Q fever. Particulate purified phase I and phase II C. burnetii rickettsiae and solubilized fractions of phase I were found to be practical antigens in the Q fever ELISA test.